Huntington's disease (HD) is an inherited neurodegenerative disease affecting 30,000 people in the United States. HD is caused by expansion of a CAG trinucleotide repeat in the first exon of the huntingtin (HTT) gene and results in translation of a protein that contains an enlarged CAG- encoded polyglutamine (polyQ) tract at the N-terminus. HD is characterized by progressive loss of neurons in the cerebral cortex and striatum during aging. This neurodegeneration results in neurocognitive decline, personality changes, chorea (involuntary movement), dementia and death. Toxicity of mutant HTT protein is caused by production of protein fragments by several protease families including caspases, calpains and matrix metalloproteinases (protein degrading enzymes). These fragments begin to accumulate and aggregate in neurons during aging and lead to the mutant HTT toxicity that causes HD. The objective of this project is to understand which fragments of HTT are the most toxic and what causes this toxicity. To answer these questions, we generated five transgenic mouse models that express an individual HTT proteolysis fragment to characterize the contribution of each proteolysis fragment to HD-related neurodegeneration. We have identified two fragments that cause accelerated neurodegeneration and have accumulated data suggesting that this neurodegeneration is caused by accumulation of HTT fragments in protein complexes that contain wild-type HTT. The aims of this project are to identify proteins that are lost or added to the wild-type HTT complex as HTT fragments accumulate in it, then to test these proteins for their role in HTT fragments' toxicity. Our long-term goal is to identify proteins that work with HTT fragments to cause the neurotoxicity in HD. Understanding which proteins work with HTT fragments to cause toxicity will give us valuable insights into the mechanisms of HD and may be relevant for other aging- related neurodegenerative diseases involving toxic protein accumulation including Alzheimer's disease (AD), Parkinson's disease (PD) and the eight other known polyQ diseases. Finally, identification of novel proteins and pathways involved in neurotoxicity in HD will enable us to develop new drugs to slow or stop progression of neurodegeneration in HD patients. PUBLIC HEALTH RELEVANCE: The therapeutic aim of our research is to mitigate the toxicity of mutant huntingtin (HTT) protein in Huntington's disease (HD), either by preventing cleavage of full-length mutant HTT protein by key proteases or by modulating interaction or levels of proteins through which HTT fragments act. Understanding the mechanisms by which HTT fragments cause toxicity in neurons has potential broader implications as several other neurological disorders associated with accumulation of toxic proteins during aging, including Alzheimer's disease (AD), Parkinson's disease (PD) and the eight other known polyQ diseases. The research outlined in this grant application has implications in the reduction of the burden of neurodegenerative illness, increasing quality of life and extending the healthy years of life.